Surface Area Retention Research Articles

Electrochemical and Morphological Studies of Ceramic Carbon Electrodes for Fuel Cell Systems

Ceramic carbon electrodes were prepared via polymerization of silane materials containing sulfonic acid groups in the presence of Pt/Vulcan XC72. Cyclic voltammetry and electrochemical impedance spectroscopy were used to investigate the effects of sulfonic acid functionalities on platinum utilization and proton conductivity, as well as CCE microstructure. It has been determined that incorporation of small amounts of sulfonated silane to the CCE structure can lead to a profound enhancement of proton conductivity despite decreases in BET surface area. Retention of surface area was achieved through combination of sulfonated and unsulfonated organosilane precursors. These composite samples show improved performance during fuel cell testing than for CCEs prepared using unsulfonated precursors.

Influence of an osmotic pre-treatment on structure-property relationships of air-dehydrated tomato slices

The influence of an osmotic step and the syrup composition on the chemical–physical properties, drying kinetics, structure collapse and colour changes of air dehydrated tomato slices, was studied. Sugar solutions, with reduced sweetening capacity with or without CaCl 2 added, were used. During air dehydration, colour attributes and geometric features were evaluated by image analysis, and an electronic nose was used to monitor the changes in aroma profile. The osmotic pre-treatment increased drying rate during the first falling rate phase of the air dehydration step but in the last phase reduced it. Sugar enrichment improved colour stability during subsequent air dehydration and helped to slightly decrease shrinkage during the first phase. The calcium addition improved surface area retention. Image analysis has proved to be a useful tool to study structure evolution during dehydration in combination with conventional analysis. The electronic nose was able to characterize the process aromatic fingerprint, helpful in understanding and parameterizing the degradative events during dehydration.

Preparation and characterization of aerogels derived from Al(OH) 3 and CrO 3

Aerogels containing both Al 2O 3 and Cr 2O 3 were prepared by the reduction, by alcohols, of a precursor salt solution derived from Al(OH) 3 and CrO 3, followed by supercritical drying in either CO 2 or ethanol. TEM analyses showed a microstructure typical of aerogels, with a connected matrix of ∼10-nm diameter particles and an open pore network. Subsequent thermal processing converts the initial aerogels to a high surface-area material comprised of Al 2O 3 and Cr 2C 3. Addition of ∼6% SiO 2, relative to Al results in an increased retention of surface area at high temperatures. Surface areas of the aerogels after supercritical drying ranged from 240 to 700 m 2/g, while after treatment at 1000 °C values ranged from 110 to 170 m 2/g. The composition which showed the greatest temperature stability was 2(0.94Al 2O 3 · 0.06SiO 2)Cr 2O 3. After treatment at 1000 °C, all a samples contained a large number of crystallites of the Cr 2O 3 phase, eskolaite, with diameters ranging from 0.5 to 1.0 μm. An additional unidentified phase may also be present. The presence of these larger crystallites leads to a lower transmittance in the near-IR due to increased scattering.

Influence of processing medium on retention of specific surface area of gamma-alumina at elevated temperature

The effects of suspending medium of water and ethanol on retention of surface area of gamma-alumina at elevated temperature were studied. The study was conducted with particular interest in the commonly used impregnation technique for stabilization of gamma-alumina. The ethanol medium of processing was compared with the more commonly employed water medium of processing while impregnating gamma-alumina for surface modification. Lanthanum and tin precursors were taken to impregnate gamma-alumina using both ethanol and water medium. Ethanol treatment retained higher surface area and increased the alpha-alumina transformation temperature. The pore size distribution of both samples showed an apparent difference and that impregnated in ethanol had larger pore size than that in water. This suggests fewer particle-to-particle contacts resulting from the improved thermal stability due to impregnation in ethanol. Evolution of finer micro-pores is observed only in the case of the ethanol-processed alumina after heat treatment at elevated temperature.

High Temperature Stabilisation of Pores in Sol-Gel Titania in Presence of Silica

Sol-gel titania was prepared by the hydrolysis and condensation reaction of acetic acid modified titanium isopropoxide. Partially hydrolysed tetraethoxy silane was further added to this precursor in order to get titania-silica mixed oxides. The silica content was varied from 1% to 50%. The gels were characterised by thermal analysis, FTIR and XRD analysis. Surface area of titania with out any additive was 320 m2/g. This got considerably reduced to 56 m2/g on tempering at 550°C for a period of one hour and further to 29 m2/g on tempering for 32 hours at the same temperature. However, in the case of silica added samples the surface area becomes constant after one hour of tempering and remains nearly at the same value even up to 32 hours. While undoped titania retains only ∼10% of its initial surface area when tempered at 550°C over a period of 32 hours, a titania-1% silica composition retains up to 26% under identical conditions. This trend further increases with increase in silica content and for a composition titania-50% silica, retention of surface area is as high as ∼80%. Untempered titania and titania-silica samples are highly micro porous in nature. On tempering, titania samples become mesoporous. However a major fraction of the micro porosity is retained in the case of the titania-silica compositions, even after tempering for 32 hours at 550°C, which also contributes to their high surface area. The results of this study are discussed.

Phosphate as promoter of zirconia for alkane isomerization reactions

Phosphate, as catalytic promoter of zirconia, has similar effects as sulfate and tungstate anions, i.e., enhanced thermal stability with high retention of surface area and higher crystallization temperature, in comparison with unpromoted zirconia. \(n\)-butane isomerization activity increases and selectivity to isobutane decreases as phosphate content increases. For \(n\)-heptane isomerization, activity and selectivity to \(i\)-C7 have maxima.

A 29Si NMR investigation of the structure of amorphous silica—alumina supports

Commercial alumina-based catalysts are often formulated with silica to provide either acidity in the support or structural stability. Hydrocracking, catalytic cracking, and mild hydrocracking catalysts generally have silica incorporated at concentrations of 20% or more; catalysts for hydrotreating, on the other hand, may have silica added at less than 5% by weight. A magic angle spinning nuclear magnetic resonance (MAS NMR) study is reported for a series of amorphous silica—aluminas using 29Si NMR to monitor local structure. The NMR of 29Si has distinctly different chemical shifts when SiO ligands are exchanged for AlO ligands, i.e. the distribution of chemical shifts can be used to determine the structure and the degree of homogeneity of SiO 2 distribution in γ-Al 2O 3 supports at SiO 2 concentrations as low as one percent. Results of such NMR measurements and correlations between method of preparation and the distribution of silica in and on alumina is presented. A particularly interesting observation is that the form of silica may play a significant role in determining the thermal stability of alumina as reflected in the retention of surface area and porosity under severe conditions and that this stability can be correlated with a given kind of silica observed by NMR.

Characterization of alumina—montmorillonite complexes

Changes in pore structure on thermal and hydrothermal treatment of three different alumina-montmorillonite complexes were investigated using nitrogen adsorption-desorption measurements, thermogravimetric analysis, adsorption of condensed aromatic molecules and acidity measurements by pyridine adsorption. The surface area retention of the alumina—montmorillonites after thermal and hydrothermal treatment increased with increasing temperature of hydrothermal treatment of the aluminum chlorohydrate solution used in the preparation of the materials. Hydrothermal treatment of the alumina-montmorillonites at 750°C for 18 h resulted in increased average layer distances in the materials primarily due to a collapse of micropores. The acidity retentions of the alumina—montmorillonites, measured by pyridine adsorption, were approximately proportional to the corresponding surface area retentions.